It is well known that starting induction motors can cause a number of problems. One of the most significant is the large amount of inrush power that can be required to start the motor. During start up phase, induction motor will draw currents, which exceed the usual current supplied to the motor when it is running at full load. The very high current demand can have a detrimental effect upon the local electricity supply, for example by causing a “brown out” due to the reduction in the supply voltage.
These problems associated with starting induction motors are set forth in published international application no. WO97/30509 (McDonald), together with a circuit and method for effectively overcoming the problem of current surges associated with starting induction motors.
The solution proposed by McDonald is to provide a stag circuit for a multi-phase motor whereby a unidirectional current derived from the mains supply is supplied between a first combination of the winding terminals of the motor. This establishes a stationary rotor flux in the rotor. At the same time as the stationary rotor flux is being established, a capacitor is charged from the mains supply. When the stationary rotor flux has been established, and when the capacitor has both charged, the supply of unidirectional current to the first combination of terminals is terminated and the charge on the capacitor is applied to a second combination of terminals. The second combination of terminals is selected to generate a stator flux at an angle between 0 and 180 degrees to the stationary rotor flux. Therefore, when the charge on the capacitor is applied to the second combination of terminals, the rotor will attempt to move into the desired position governed by the orientation of the fields. A very high voltage can be built up upon the capacitor, and therefore a correspondingly high current may be provided in the winding between the second combination of terminals, so a very significant and substantial starting torque can been applied to the motor to initiate rotation. Once rotation has been initiated in this fashion, then the mains supply can be connected to the terminals of the motor in the known way to continue the rotation of the motor.
Therefore, the starting circuit described in McDonald has the significant advantage of being capable of being arranged to provide a very high starting torque without drawing a correspondingly high surge current from the mains supply during start up.
A significant disadvantage with the circuit described in McDonald is that there is no easily measured yet accurate and reliable way of determining whether or not the machine has actually started after the charge from the capacitor has been applied to the second combination of terminals. Clearly, if the motor has not started, ie. if the rotor has not started rotating, then applying the fill mains supply to the motor can result in destruction of the motor. Such situations may often occur, for example when the rotor becomes locked or something in the circuitry is faulty so that the rotor does not start turning when the charge on the capacitor is applied to the second combination of terminals. The condition is worse with a single phase supply as there is then no natural rotation of the supply voltage.
In McDonald, the initiation of the connection of the mains supply to the motor terminals occurs when any of the following three events happens:    1. If the current delivered by the capacitor to the second combination of terminals falls to zero before a predetermined time period elapses, or    2. If the predetermined time period elapses before the current delivered by the capacitor falls to zero, or    3. If the rate of change of current from the capacitor goes to zero before the predetermined time period elapses.
Therefore, the strategy is not dependent upon any indication as to whether the rotor is turning, so there is no indication as to when it is safe to connect the supply to the motor.